Packing material for letterpress impression cylinders



Feb. 18, 1969 E. H. GRUPE ETAL PACKING MATERIAL FOR LETTERPRESSIMPRESSION CYLINDERS Filed Nov. 27, 1962 FORM A WATERLEAF PAPER SATURATEWITH A HEAT-CURABLE ELASTOMER/C ATEX HEAT CURE SATURATED PAPERSUPERCALENOER T0 APFARENT DENSITY OF BETk/EENSAND 6 LAMINATE MULTl-PLIES0F SUPER-CALENDERED PAPER WITH HEAT CURABLE ELASTOMERIC LATEX CONDITIONLAN/NATE TO MOISTURE CONTENT OF ABOUT 5% Sheet Feb. 18, 1969 I E. H.GRUPE ETAL I 3, 7

PACKING MATERIAL FOR LETTERPRESS IMPRESSION CYLINDERS Filed Nov. 27,1962 sheet 2 of 2 United States Patent 4 Claims ABSTRACT OF THEDISCLOSURE An improved packing material for letterpress impressionmaterials consisting of a unitary laminate of elastomer impregnated websof alpha-cellulose fibers. The laminate has a uniform apparent densityof about 5 to 6 obtained by heat-curing the elastomer impregnant in thecomponent webs and subsequently supercalendering the cured webs to thedesired density before laminating. After laminating, one side of thelaminate is microground to a depth less than the thickness of onecomponent web. The finished laminate has a smooth suede-like finish onthe microground side. The microgrinding removes local areas ofdisuniformity in thickness and density so that the maximum within-sheetthickness variation is less than :L.0075".

This invention relates to an improved material for packing theimpression cylinder of letterpress printing presses. It relates furtherto a method for manufacturing packing material with improved uniformityin thickness and density.

In letterpress printing, the printing member commonly is a rotatingcylinder on which raised type or other printing means are mounted. Theweb to be printed passes through a printing couple formed by therotating printing member and a coacting impression cylinder. Theprinting member and impression cylinder are held in spaced relationshipby cooperating peripheral bearer strips. The bearer strips of theimpression cylinder form the lateral boundaries for a peripheralundercut area of the cylinder. The press is made ready for printing byfilling in this un dercut area to a level slightly above the bearerstrip surface with layers of sheet stock which include a resilientpacking material. Filling in the undercut area to the desired level iscalled makeready and provides the impression cylinder with a suitableyielding surface to hold the web firmly against the inked portion of theprinting member and effect ink transfer from plate to web.

Impression cylinders on presses manufactured 50 years ago were commonlyundercut about 0.060" to 0.070" for packing. Today, very few pressesused in general shop work, as well as magazine printing, have bearercuts more than 0.040", and the newer presses have reduced the undercutstill more to about 0.035". Thus, thinner packings place a greaterresponsibility on the plate maker to produce higher quality printingplates of uniform levelness, and at the same time additionalresponsibility is placed on the pressman to obtain greater precision inpacking the impression cylinder and underlaying the plates duringmakeready. As a result, the printers are requiring manufacturers ofpacking materials to produce packing materials with more uniformthickness and density.

In addition to uniform thickness and density, the packing material mustnot mat down or emboss during use. It must have the quality of quickrecovery from the high pressures imposed by the printing press in orderthat each successive impression on the packing will providesubstantially the same results as its predecessor, especially whensuccessive colors are used in multicolor printing. For exice ample, on acommon impression cylinder used in multicolor work, the length of timebetween successive color application on a high speed press may be aslittle as of a second. Thus, the ideal packing material should havealmost instantaneous recovery, or at least provide a uniform pressurefor each rapidly succeeding impression.

It is therefore an object of this invention to provide improvedletterpress impression cylinder packing material having suitablerecovery properties as well as more uniformity in thickness and densitythan it has hitherto been possible to obtain. It is an additional objectto provide a method for manufacturing such improved packing material.

Additional objects and advantages of the invention will become apparentfrom the following detailed description of certain preferred embodimentstogether with the accompanying drawings, in which:

FIG. 1 is a flow diagram showing the sequence of steps involved inproducing the preferred embodiment of the packing material of thisinvention.

FIG. 2 is a greatly enlarged cross section of one embodiment of thepacking material of this invention in an intermediate stage, i.e., aftersupercalendering and lamination.

FIG. 3 shows the same cross sectional area of the packing material as inFIG. 2, after microgrinding one side.

FIG. 4 is a simple cross sectional view of a standard microgrindingapparatus employed in grinding the packing material.

As noted above, more and more printers are requiring that the materialsemployed to build up impression cylinder packing have better uniformityin thickness and density in order that higher quality printing may beobtained. Experience has shown that the sheets making up the resilientlayer in impression cylinder packings are the elements which giveprinters the most trouble in the way of thickness and density variation.This invention is directed to providing a resilient packing material inwhich the maximum thickness variation within the resilient packingmaterial in the standard thicknesses required for packing is reduced toless than 20.00075, and preferably to less than $000025", withoutdestroying other desirable characteristics in such sheets.

Before describing the improved packing material of this invention, abrief description of the arrangement of packings on the undercut portionof impression cylinders appears to be in order.

In the packing, or makeready, of impression cylinders several types ofsheet material are ordinarily used. The surface, or top sheet, iscommonly called a tympan. The tympan is placed over an underlayment, themajor thickness of which comprises a less dense resilient materialdesigned to yield firmly under the pressures imposed by the printingplates. The impression cylinder packing may be built-up further to thedesired thickness by positioning additional hard sheets of regulartympan, or manila, underneath the thicker resilient material in directcontact with the impression cylinder.

Tympan is normally a tough, even-calipered, hard manila colored paperusually treated with oil. Another type of tympan is made with a hardcoating of thousands of microscopic ceramic beads. Tympan ranges inthickness from about 0.003 to about 0.012". The most common thicknessesemployed are in the range of 0.006" to 0.010".

The tympan sheet is severely calendered during manufacture to harden thesheet and to reduce within-sheet caliper or thickness variations.However, even with hard calendering, actual caliper variations are inthe neighborhood of 10% of the total thickness, while density variationsare magnified rather than improved. Inherent limitations in the papermanufacturing process produce thick and thin areas in the formed sheetand militate against reducing thickness or density variations below theaforesaid 10%. Similar limitations apply to the other paper sheets usedin building up the thickness of the packing to the desired level.Efforts to reduce the thickness and density variations of any of theelements making up the packing have been only partially successful up tonow.

Most of the effort spent in attempting to solve this problem has beendevoted to work on the resilient element in the packing combinationsince this element comprises the major thickness of the packing, i.e.,from .020" to 4030", hence usually has proportionately greaterWithinsheet thickness variation, and is the major contributor tovariations in packing thickness.

One improvement in this direction has been the recent introduction intothe market of a resilient packing material comprising elastomerimpregnated cellulose fiber sheets laminated together and heat-cured ina high pressure press. Such material and its method of manufacture isdescribed in US. Patent 3,053,718, Hechtman et al., issued Sept. 11,1962. By laminating several sheets together the thick and thin spots aremore or less average-d out, and by applying high pressure andsimultaneously curing the laminate, thickness variations were reducedfrom over 20% to the neighborhood of 10%. This brought the averagethickness variation of the packing combination down but more improvementwas desired.

Attempts were made to laminate at higher pressures, but while thisreduced thickness variations somewhat more, density of the materialincreasedabove the desirable limits, and the resulting packing was toohard for optimum results. However, even with higher pressures, thewithin-sheet thickness variation was never reduced to less than 1.00075"in the thicknesses commonly employed.

It has been established that the preferred apparent density (basisweight of the sheet in pounds per 17 x 22" 500 sheet ream divided bythickness or caliper in mils) is between about 5 and 6. Packing materialwith apparent densities lower than this range are too soft causingundesirable embossing of the printed sheet and tend to mat down aftershort use, while material having densities above this range tend to betoo hard and often cause ink smearing and other undesirable effects.

In accordance with this invention, it is now possible to produce anelastomer impregnated cellulose fiber material as a resilient packingmember in which the withinsheet thickness variation is reduced tosubstantially less than 1.00075", and preferably to less than 10.00025",or in the thicknesses now commonly used for packing impressioncylinders, to less than 2%.

In one method of producing the improved packing material of thisinvention, a waterleaf paper comprised primarily of alpha-cellulosefibers is saturated with from 50 to 70 parts by weight of an aqueousdispersion of a heat-curable elastomer. The saturated material is thenheat-cured for several hours at elevated temperatures. The curedmaterial is then brought up to the preferred apparent density of betweenabout 5 and 6 by passing it through several pressure nips of asupercalender. The number of nips and pressures employed in thesupercalender are not critical and may be readily adjustable to obtainthe desired apparent density of from about 5 to 6. This supercalenderingoperation also reduces gross within-sheet thickness variations to about10% of the total thickness. After supercalendering, several plies of thematerial are laminated together by interposing a glue line comprising anaqueous dispersion of a heatcura'ble elastomer between plies and windingup the plied material. Moisture in the wet glue line application assistsin conditioning the laminate to a moisture content of about 5% in whichcondition the laminate exhibits its best dimensional stability for laterprocessing and use.

After supercalendering and lamination, the laminated material was foundto have a within-sheet thickness variation of about 1.0026". While theaverage apparent density of the material at this stage is within theoptimum range of resiliency for high quality printing, and the productwas so used without further processing, it was found that localdisuniformities in the printing pressures during use caused by similarlocal disuniformities in thickness and density detracted from printingquality to some extent.

It has now been found that most of the local disuniformities inthickness of the packing material can 'be eliminated by carefullygrinding off a portion of one surface in an amount less than thethickness of one ply. Conventional microsanding apparatus called amicrogrinder, commonly used in processing fine glove leather and ingrinding down decorative laminates to provide a level surface on theattachment side, is readily adaptable for this purpose. One such type ofapparatus is manufacture-d and sold under the tradename Lightning Micro-Grinder by the Curtin-Hebert Co., Inc. of Gloversville, N.Y. A simplecross section of such an apparatus is shown in FIG. 4 for purposes ofillustration and is described hereinafter. It is also noted that inorder to satisfactorily grind the packing material as desired, themachine must be carefully adjusted to minimize mechanical variations.Such adjustments are also further described hereinafter.

When a grinding operation was first contemplated, it was believed thatit would be necessary to grind down both sides of the laminated materialto remove disuniformities or high spots from both sides. Surprisingly,it was discovered that when the grinding apparatus is properly adjustedand only one side of the sheet ground down in an amount suflicient toremove between about 5% to 20% of the total thickness, but less than thethickness of one component ply, maximum within-sheet caliper variationis reduced to less than 10.00025", which in the thicknesses now comonlyused for packing amounts to about 2%. This reduction in within-sheetcaliper variation is a vast improvement over any packing materialhitherto produced. Thus, for example, if the original thickness of thelaminate was .026" and after grinding was .022", the within-sheetthickness or caliper variation was less than 10.00025", i.e., 11% or atotal variation of about 2% of the total thickness, compared to athickness variation of about .0026" before grinding. While a withinsheetthickness variation of less than 10.00025" is achieved in the preferredform of the invention, it is intended to cover any improvement inreducing the degree of disuniformity in thickness of laminated packingmaterial by the microgrinding technique. For example, anything less than1.00075 in within-sheet thickness variation is an improvement over priormaterials.

The flow diagram of FIG. 1 briefly outlines the sequence of stepsemployed to produce the preferred embodiment of the improved packingmaterial.

FIG. 2 shows, in greatly enlarged cross section, the packing materialafter supercalendering and lamination, but before grinding. Individualplies indicated at 5, 6 and 7 show thick and thin areas. These thick andthin areas are inherent in the papermaking process and are present nomatter how good the sheet formation may be. Thickness variations, causedby random mating of such thick and thin areas during lamination, arepresent in the material at this stage and are shown in the areas markedby opposing arrows '8, 9 and 10. To the unaided eye, the material atthis stage appears to be uniform in both thickness and density. However,by applying calipers to randomly selected areas of the sheet, it isfound that within-sheet thickness actually varies considerably. Forexample, assuming that the average thickness of the laminated sheet is.026", a measurement taken at point 8 shows the actual thickness in thatarea to be about .031; at point 9 about .029"; and at point 10 about.024". Thus, measurable thickness variations are in the neighborhood of1.0026".

After microgrinding, layers 5' and 6 are relatively unchanged as shownat 5a and 6a, but a major portion of layer 7 is removed, as shown at 7a,and the configuration of the surface of 7a is changed to reflect thecontour of the surface of layer 5a. Apparently when the sheet is passedthrough the microgrinder, more material is removed from the thickerareas than from the thinner areas thus minimizing both density andthickness variations. In the microground sheet, caliper measurementstaken at 8a, 9a and 10a are relatively uniform. Again, assuming that anaverage of .004" of material was removed by the grinding, measurementstaken at 8a, 9a, and 10a now show the material to be a uniform .022"thick with variations of less than 1.00025". To the unaided eye, thefinished material looks uniformly thick as before, but in themicroground material this uniform appearance is an actuality and may beaffirmed by random caliper measurements.

As noted previously, it first appeared that in order to obtain in thepacking material a thickness uniformity within the desired limits, itwould be necessary to remove high spots by grinding both sides of thelaminated material down to a plane parallel to the lowest spots.However, the unexpected mirror conformation of the contours on bothsides of the sheet to each other when only one side was microgroundproved the anticipated double grinding unnecessary.

For a better understanding of the invention, a simple cross sectionalview of a standard microgrinding apparatus is illustrated in FIG. 4-. Itcomprises a work table 11, over which the packing material 12 is fedunder guard 13 through feed rolls 14 and 15, and guide plates 16 and 17,into grinding nip 18 formed by abrasive paper wrapped grinding cylinder20 and vertically adjustable pressure roll 21. The ground materialpasses from the grinding nip 18 over guide plate 26 through dischargerolls 23 and 24. Pressure roll 21 and feed rolls 14 and arefriction-ally driven by drive roll 22. Discharge rolls 23 and 24 arealso frictionally driven by intervening roll 25 and pressure roll 21.Grinding cylinder is independently driven and turns in a directionopposite to the movement of the material being ground with speedadjustable as desired. Depth of cut may be controlled by adjusting thespeed of grinding cylinder 20 and the height of pressure roll 21. Innormal operation, the material is fed forward at the rate of about perrevolution of the grinding cylinder 20. It its understood that variouschanges in speed and mode of operation may be made without departingfrom the spirit of this invention.

Because of the close tolerances required in the specification of thefinished material, it was found necessary to make certain mechanicaladjustments in the microgrinder to eliminate operating variables. Forexample, the heat from grinding and from pressures exerted on thebearings of the grinding cylinder caused the cylinder to expanddisproportionately from center to end. The microgrinder is normallyequipped with a cooling water system to keep the cylinder cool andostensibly reduce disproportionate expansion. However, it was found thatthe cool ing water must be recirculated and its temperature controlledto obtain optimum results. Also, it was found that heat generated by thebearings was transferred to the cylinder ends causing a gradient intemperature whereby the ends expanded more than the center, and causedproportionately more material to be removed from the edges of the sheetas it was being ground. This is overcome by water-jacketing the bearingends, and recirculating all the cooling water through a storage tank inwhich the water temperature is controlled at slightly above roomtemperature.

Also, the grinding roll and pressure roll were found to be slightlyeccentric or out of round, which adversely affected the grindingresults. This is corrected by first grinding the pressure roll tosubstantial roundness, utilizing the abrasive paper wound grindingcylinder and running the grinder under conditions such as are used inthe regular grinding operation. Then, the eccentricity is removed fromthe grinding cylinder, by stripping it of abrasive paper, turning thepaper grit side up, and manipulating the abrasive paper between thepressure roll and grinding cylinder until the latter too is ground tosubstanial roundness.

If these truing adjustments are not made before grinding the packingmaterial, the material will be ground down to a maximum within-sheetthickness variation of about $00075". While this latter is animprovement over the unground sheet, it is nowhere in the range ofimprovement which can be obtained when proper adjustments, in theequipment are made as described herein.

Best results are obtained when the material is microground while it hasa moisture content of about 5%, since the laminated product was found tohave optimum dimensional stability at such moisture content. It was alsofound that the best method of grinding was first to remove the majorportion of the material on the microgrinder by employing a coarsesandpaper of about 120 grit silicon carbide on the grinding drum on thefirst pass through the grinder, followed by a second pass through thegrinder employing a finer 220 grit silicon carbide sandpaper to grindthe sheet down to the desired caliper. The sheet to be ground should bepassed through the grinder in its machine, or grain, direction. Equallygood results can be obtained by employing the fine grit abrasive paperfor the entire operation, using more than two passes through thegrinder. However, in the interest of time and process economy theemployment of a first coarse grinding step to remove the major portionof material is preferred. While the sheet may be run through the grinderin the same direction for each pass and still obtain the desiredresults, it has been found that if the direction is reversed on thesecond pass, optimum results are obtained.

The following example will more clearly illustrate one specificembodiment of the invention. It is given by way of illustration only andis not intended to limit the scope of the invention.

A water-leaf saturating paper having a basis weight of about 27 lbs. per17" x 22" 500 sheet ream was formed on a conventional Fourdrinier papermachine. The furnish comprised about 15% bleached hardwood sulfite pulpand bleached spruce kraft pulp. Both pulps were caustic extracted andthus consisted primarily of alpha-cellulose fibers to provide the papersheet with the most uniform formation possible.

The paper was saturated with a 36% aqueous dispersion of a heat-curablesynthetic elastomer comprised of a butadiene-acrylonitrile copolymercompounded with selected curing agents. In this specific instance, thesaturant consisted of parts by weight of a copolymer of 68 /2 percentbutadiene and 31 /2 percent acrylonitrile, 5 parts phenol-formaldehyderesin, 2 parts sulfur, 2 parts butyl zimate, 3 parts zinc oxide and 3parts non-ionic soap. Actual pickup of saturant by the paper was about65 parts of saturant solids per hundred parts of bone dry fiber.

The saturated paper was cured at 225 F. for about 5 /2 hours. Aftercuring, the treated paper was measured and found to have an averagethickness of about .0101" and an apparent density of about 4.5. Thecured paper was then run through an eight nip supercalender atsufficient pressure to give the paper an apparent density in the rangeof 5 to 6. In this example, the supercalender condensed the paper to anaver-age thickness of about .0079" and an apparent density of about 5.4.Within-sheet caliper variation was in the range of about 10%. Threeplies of the supercalendered paper were then laminated by interposingbetween the plies a coating of about 9 lbs. per ream of a 60% aqueousdispersion of a butadiene-acrylor nitrile copolymer and pressing theplies together on a wind-up roll. The laminated paper was thenconditioned to a moisture content of about 5% The laminated material wasmeasured and found to have an average thickness of .02595" and anapparent density of about 5.6. Within-sheet caliper variation was in therange of 1.0026", i.e., about 110%.

The laminated and conditioned material was then microground on one sideby passing it through a microgrinder adapted in the manner described,first in one direction using a 120 grit silicon carbide paper on thegrinding cylinder, and then in the opposite direction using a 220 gritsilicon carbide paper on the grinding cylinder. In the first pass,approximately .004 of material was removed. In the second pass about.001 of material was removed. The microground side had a smoothsuede-like finish.

The resulting material had an average caliper of 021%", an apparentdensity of 5.7, and a within-sheet caliper variation of less than1.00025.

The finished material was installed as the packing material on a highspeed four color press using the following makeready assembly.Conventional unoiled manila tympan was first applied to the impressioncylinder in a thickness of about .010". Then the packing material ofthis invention was applied. A top sheet of oiled hard manila tympancompleted the packing. The total height over the impression cylinderbearers was about .002" to establish the printing pressure.

The packed press was run for over one million impressions with excellenthalf tone reproductions and markedly improved trapping of color in heavycoverage areas when compared to results obtained employing older typepackings. In addition, less makeready time was required.

It was also found that after shorter runs of 200,000 to 500,000impressions, the improved packing material of this invention could bereused by wiping the surface with kerosene and recovering the packingwith a new conventional manila tympan sheet.

It has also been found that when foreign material passed through theprinting nip causing batters of sufficient severity to penetrate thetympan sheet and damage the packing material, repairs could be made bysimple local application of a small amount of glycerine. More seriouslydamaged packing sheets may be replaced with complete assurance that thereplacement sheet will equal the previous sheet within at least onethousandth of an inch, thus cutting down on potential time-consuming andexpensive repacking time.

While the emphasis throughout this specification describes the preferredembodiment of the packing material as being of laminated construction,it is possible to make a less satisfactory, but still serviceable,material by forming the base sheet sufficiently thick so that only oneply is required. Suitably thick sheets can be made on conventionalcylinder or board machines. Even though formation on these machines isusually not as uniform as can be obtained on Fourdrinier machines, whensuch a sheet is saturated, cured, supercalendered, and microground asherein described, most of the improved characteristics will be attained.

The preferred elastomer is a butadiene-acrylonitrile copolymercontaining from about 55 to 80 percent butadiene and about to 45 percentacrylonitrile. Other elastomers which may be used included materialssuch as butadiene-styrene copolymers, natural rubber, polychloroprene,copolymers of an alkyl acrylate and an unsaturated carboxylic acid,copolymers of an alkyl acrylate, an unsaturated carboxylic acid and analkyl methacrylate, polymerized methyl, ethyl or butyl acrylate, ormethyl, ethyl, or butyl acrylate copolymerized with acrylonitrile orethyl, methyl, or butyl methacrylate, and the like. The hardness of thesheet can be controlled by judicious selection of the saturant from thisgroup.

Impregnating is usually done from an aqueous dispersion althoughsolutions in suitable solvents may also be used. The amount of solids inthe impregnating medium may range from about 20 to 40 percent. Thedesirable degree of retention is from about 50 to 70 parts by weight ofsolids per 100 parts of dry fiber althought he amount of retention mayrange from about 35 to 140 parts of solids per 100 parts of dry fiberand still be satisfactory for some uses. Alternatively, the elastomermay be added in the beater and precipitated on the fibers before thesheet is formed. In this case, the saturating step is omitted. However,the elastomer must still be cured in situ before supercalendering.

In addition to the elastomer, the impregnating mixture usually containsspecial additives to enhance certain specific properties. Theseadditives include curing agents such as Zinc oxide, sulphur, zincdibutyl dithio-carbamate or dicumyl peroxide; resins such as phenolformaldehyde; fillers such as clay, carbon black, and one or more of thecarbonates, dyes and pigments.

It is understood that overall thickness of the laminated packingmaterial may be varied to meet the needs and requirements of theprinting industry. In any event, in carrying out the preferredembodiment of the invention as taught herein, the variation in thicknesswill never exceed 1.00025" which in the thickness currently preferred bythe printers is less than 12%.

The unusually low within-sheet thickness variation obtained in thepacking material as described herein enables the printer to packimpression cylinders to a lesser height over the bearer strips thanhitherto possible so that satisfactoryprinting runs may be made withless pressure in the printing couple. This allows the printer toexercise better control over the printing process, enables the use ofinks with closer tackiness values, and protects against serious damagefrom batters caused when extraneous material or extra thick paper is inadvertently passed through the nip during printing.

We claim:

1. An improved packing material for letterpress impression cylinders,said material being of uniform density and thickness and comprising aunitary laminate of cellulosic fiber webs impregnated with and bonded byan elastomer which has been heat-cured in situ, said fibers consistingprimarily of alpha-cellulose fibers and said elastomer comprising abutadiene-acrylonitrile copolymer containing from about 55 to percentbutadiene and about 20 to 45 percent acrylonitrile, said laminate andits component webs having an apparent density of about 5 to 6 obtainedby heat-curing the elastomer impregnant in the component Webs andsubsequently supercalendering the cured webs to the desired densitybefore lamination, one side of said laminate has a smooth suede-likefinish obtained by microgrinding said laminate to a depth less than thethickness of one component web and suificient to remove local areas ofdisuniformity in thickness and density, said microground laminate havinga maximum Within-sheet thickness variation of less than 1.00075".

2. The improved packing material of claim 1 in which the elastomer ispresent in the amount of about 35 to 140 parts by weight per parts ofdry fiber.

3. An improved packing material for letterpress impression cylinders,said material being of uniform density and thickness and comprising aunitary laminate of cellulosic fiber webs impregnated with and bonded byan elastomer which has been heat-cured in situ, said fibers consistingprimarily of alpha-cellulose fibers and said elastomer comprising abutadiene-acrylonitrile copolymer containing from about 55 to 80 percentbutadiene and about 20 to 45 percent acrylonitrile, said laminate andits component webs having an apparent density of about 5 to 6 obtainedby heat-curing the elastomer impregnant in the component webs andsubsequently supercalendering the cured webs to the desired densitybefore lamination, one side of said laminate has a smooth suede-likefinish obtained by microgrinding said laminate to a depth less than thethickness of one component Web and suifi- 9 cient to remove local areasof disuniformity in thickness 3,053,718 and density, said microgroundlaminate having a maxi 2,202,020 mum within-sheet thickness variation ofless than 2,822,855 1200025". 3,026,217

4. The improved packing material of claim 3 in which the elastomer ispresent in the amount of about 35 to 140 parts by weight per 100' partsof dry fiber.

References Cited UNITED STATES PATENTS 5 JACOB H. STEINBERG, PrimaryExaminer.

R. J. ROCHE, Assistant Examiner.

US. Cl. X.R. 10 161-244, 251, 401; 162-169, 288; 117-64

